Nitrophenyl acetate, imidazole catalysed hydrolysis

Fig. 11.6 (A) Spectroscopic detection of acetylimidazole in the imidazole-catalysed hydrolysis of 4-nitro-phenyl acetate at pH 5 (D) and at pH 6 (E) curves are calculated from data in reference [12] and the curve for the acetate ion product (C) is for pH 5. (B) Reaction of imidazole with 4-nitrophenyl acetate (k) and the hydrolysis of acetyl imidazole (k2) curves constructed from data in references [13] and [12].

Figure 11.6A illustrates the effect of a change in the ratio of ki/k2 for the imidazole-catalysed hydrolysis of 4-nitrophenyl acetate. When the pH is changed from 5 to 6, the maximal concentration of the intermediate increases. The pH profile in Fig. 11.6B provides a graphic illustration of the range of pH within which an intermediate can be detected this is between the cross-over pH values of about 5 and 10 for these conditions. The range illustrated is for 0.1 M imidazole and could be extended because k is dependent on the concentration of imidazole whereas k2 is not. 

Radical polymerization of (14) and the subsequent removal of the benzyl group produces water-soluble polymers containing imidazole-4-carbohydroxamic acid residues. The hydrolysis of 4-nitrophenyl acetate catalysed by this polymer proceeds faster than that catalysed by imidazoles or carbohydroxamic acid but slower than that catalysed by the monomeric analogue. " The hydrolysis of 4-nitrophenyl acetate is catalysed by the bifunctional hydroxamic acid (IS) and... 

Water pooled in reverse micelles enhances the rate of the imidazole catalysed hydrolysis of 4-nitrophenyl acetate. The rate of acylation of amines in apolar solvents is enhanced more by cationic micelles than by bis(2-ethylhexyl) sodium sulphosuccinate, and may be treated by the pseudophase model. ... 

Bruice and Sturtevant, (1959) and Bruice, (1959) found extremely facile intramolecular nucleophilic attack by neighbouring imidazole in the hydrolysis of p-nitrophenyl 7-(4-imidazoyl)butyrate [19]. The rate constant for imidazole participation (release of p-nitro-phenolate) in this reaction is nearly identical with the rate constant for a-chymotrypsin catalysed release of p-nitrophenolate ion [190 min in equation (11) at pH 7 and 25°] from p-nitrophenyl acetate. Comparison of the rate constant for intramolecular imidazole participation to that for the analogous bimolecular reaction (imidazole attack on p-nitrophenyl acetate) (s" /m s )... 

In general, mechanistic evidence for a reactive intermediate from trapping experiments needs to be linked to arguments against the introduction of an alternative pathway from the reactant, i.e. to show that an intermediate really has been trapped, not the reactant. A classic case is the hydrolysis of 4-nitrophenyl acetate catalysed by imidazole. The mechanism is nucleophile catalysis and the intermediate (N-acetylimidazolium cation) was trapped by aniline (to give acetanilide) with no kinetic effect, i.e. the aniline does not react directly with the substrate [51]. 

A great variety of chemical reactions can be advantageously carried out in microemulsions [860-862]. In one of the first papers in this field, Menger et al. described the imidazole-catalyzed hydrolysis of 4-nitrophenyl acetate in water/octane microemulsions with AOT as an anionic surfactant [=sodium bis(2-ethyl-l-hexyl)-sulfosuccinate] [864]. The solubilized water, containing the imidazole eatalyst, is confined in spherical pools encased by surfactant molecules, which have only their anionic head groups (-SOb ) immersed in the aqueous droplets. When the ester, dissolved in water-insoluble organic solvents, is added to this water/octane/AOT/imidazole system, it readily undergoes the catalysed hydrolysis under mild reaction conditions (25 °C). 

The rate of hydrolysis of 4-nitrophenyl caproate catalysed by poly(ethyl-enimines) containing imidazole residues is markedly enhanced in the presence of divalent metal ions. The order of effectiveness is Cu > Co > Zn > Ni > Mn", with copper(ll) increasing the rate ca. 20-fold. The acceleration by the metal ion is much greater in the presence of acetate and chloride ions than in the presence of perchlorate ions. ... 

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